Field of the Invention
The present invention relates to an electron microscope and method of operating it.
Description of Related Art
Generally, electrons released from a field-emission electron gun contain a fluctuation portion representing several percent of the total amount, because gas is adsorbed onto the emitter surface and adsorbed gas and ions migrate, varying the work function of the metal surface and because ion bombardment varies the metal surface morphology. Therefore, where a field-emission electron gun is used in a scanning transmission electron microscope (STEM), emission noise on the image is removed by placing a detector for noise cancellation in the electron optical column, detecting near electrons forming a probe to thereby form an electron signal, and dividing a signal released from the sample by the electron signal. This noise cancellation technique is disclosed, for example, in JP-A-5-307942.
FIG. 13 shows the configuration of a scanning transmission electron microscope (STEM), 101, having a general noise canceling function. This microscope 101 has an electron optical column 110 in which a cold field-emission gun (CFEG) 111, a noise canceling aperture 112, a lens 113, another lens 114, a detector 115, a preamplifier circuit 120, an amplifier circuit 130, and other components are housed. The electron beam released from the CFEG 111 is partially cut off by the noise canceling aperture 112 and then converged onto a sample A by the lens 113. The beam transmitted through the sample A passes through the lens 114 and is partially detected by the detector 115. The output signal from the detector 115 indicative of an image is the product of an emission current I1 hitting the sample A and the brightness component S of the sample A, i.e., I1×S. The emission current I1 hitting the sample A and an emission current I2 detected by the noise canceling aperture 112 are in a proportional relationship (I1=n×I2). An offset component is added to the image signal (I1×S) by the preamplifier circuit 120 and the resulting sum is amplified by a factor of Gp and further amplified by a factor of Ga by the amplifier circuit 130. The emission current I2 detected by the noise canceling aperture 112 is amplified by a factor Gn by a noise detection circuit 140. Where the noise canceling function is not used, the output signal from the amplifier circuit 130 is made to bypass a noise canceling circuit 150, and is computationally processed in a given manner by a central processing section (CPU) 160. Then, the signal is sent to a personal computer (PC) 102, where an STEM image of the sample A is displayed on its display screen. Where the noise canceling function is used, the noise cancelling circuit 150 produces the difference between the output signal from the amplifier circuit 130 and the offset component added by the preamplifier circuit 120. This difference is divided by the output signal from the noise detection circuit 140. Consequently, the emission noise contained in the image signal is removed. The image signal free of the emission noise is computationally processed in a given manner by the processing section (CPU) 160 and then sent to the personal computer (PC) 102, where an STEM image of the sample A free of the emission noise is displayed on its display screen of the PC 102.
FIG. 14 shows one specific example of the configuration of signal processing circuitry used when the noise canceling function is not used. As shown, when the noise canceling function is not used, STEM imaging is carried out fundamentally by adjusting two parameters, i.e., contrast and brightness. The contrast is a gain added to the image signal to adjust the light-dark condition. The brightness is a DC voltage added to cancel the offset component of the image signal. In the example of FIG. 14, the image signal S×I1 is obtained from the detector 115 by adjusting the contrast and brightness B is added to the image signal S×I1 by an adder 122 in the preamplifier circuit 120. The resulting sum signal is amplified by a factor of Gp by an amplifier 124. Accordingly, the output signal from the amplifier 124 is given byV11=Gp×(S×I1B)  (A)
The output signal V11 from the amplifier 124 is amplified by a factor of Ga by an amplifier 132 in the amplifier circuit 130. Using Eq. (A) above, the output signal V12 from the amplifier 132 is given byV12=Ga×Gp×(S×I1+B)  (B)
The analog output signal V12 from the amplifier 132 is converted into digital form by an A/D converter 162 in the processing section 160, then averaged or otherwise arithmetically processed, and sent to the PC 102 shown in FIG. 13.
FIG. 15 is a diagram showing a specific example of configuration of signal processing circuitry used when the noise canceling function is used. As shown, when the noise canceling function is used, too, the output signal V12 from the amplifier 132 is given by the above Eq. (B). An amplifier 151 of the noise cancelling circuit 150 gives a gain equal to the product of the gain Gp of the amplifier 124 and the gain Ga of the amplifier 132 to the brightness B to cancel the brightness B added by the preamplifier circuit 120. A subtractor 152 subtracts the resulting sum from the output signal V12 from the amplifier 132. Using Eq. (B), the output signal V13 from the subtractor 152 is given by
                                                                        V                13                            =                            ⁢                                                Ga                  ×                  Gp                  ×                                      (                                                                  S                        ×                        I                        ⁢                                                                                                  ⁢                        1                                            +                      B                                        )                                                  -                                  Ga                  ×                  Gp                  ×                  B                                                                                                        =                            ⁢                              Ga                ×                Gp                ×                S                ×                I                ⁢                                                                  ⁢                1                                                                        (        C        )            
The emission current I2 detected by the noise canceling aperture 112 is converted into a voltage by an amplifier 142 in the noise detection circuit 140 and amplified by a factor of Gn. Therefore, the output voltage from the amplifier 142 is given byV14=Gn×I2  (D)
The output signal V13 from the subtractor 152 is applied to the input (X) on the numerator side of a dividing circuit 154. The output signal V14 from the amplifier 142 is applied to the input (Y) on the denominator side of the dividing circuit 154. Using Eqs. (C) and (D), the output signal V15 from the dividing circuit 154 is given by
                              V          15                =                              X            Y                    =                                                    V                13                                            V                14                                      =                                          Ga                ×                Gp                ×                S                ×                I                ⁢                                                                  ⁢                1                                            Gn                ×                I                ⁢                                                                  ⁢                2                                                                        (        E        )            
To perform the aforementioned subtraction from the output signal V12 from the amplifier 132 using the subtractor 152 of the noise canceling circuit 150, an amplifier 155 gives a gain equal to the product of the gain Gp of the amplifier 124 and the gain Ga of the amplifier 132 to the brightness B. An adder 156 adds the gain to the output signal V15 from the dividing circuit 154. Thus, the output signal V16 from the adder 156 is given by
                                                                        V                16                            =                            ⁢                                                                    Ga                    ×                    Gp                    ×                    S                    ×                    I                    ⁢                                                                                  ⁢                    1                                                        Gn                    ×                    I                    ⁢                                                                                  ⁢                    2                                                  +                                  Ga                  ×                  Gp                  ×                  B                                                                                                        =                            ⁢                                                S                  ×                                                            Ga                      ×                      Gp                                        Gn                                    ×                                                            I                      ⁢                                                                                          ⁢                      1                                                              I                      ⁢                                                                                          ⁢                      2                                                                      +                                  Ga                  ×                  Gp                  ×                  B                                                                                        (        F        )            
The analog output signal V16 from the adder 156 is converted into digital form by the A/D converter 162 in the processing section 160, then averaged or otherwise arithmetically processed, and sent to the PC 102 shown in FIG. 13.
Substituting I1=n×I2 into Eq. (F) results in
                              V          16                =                              S            ×                                          Ga                ×                Gp                            Gn                        ×            n                    +                      Ga            ×            Gp            ×            B                                              (        G        )            
Any of the emission currents I1 and I2 containing emission noise does not exist in the right side of Eq. (G). Consequently, where the noise canceling function is used, values proportional to the brightness component S of the sample A that the operator wants to image in the same way as where there is no emission noise can be obtained.
If the dividing circuit 154 performs a division operation without removing the brightness B added by the preamplifier circuit 120 and without mounting the amplifier 151 or subtractor 152, the output signal V15 from the dividing circuit 154 is given by
                                                                        V                15                            =                            ⁢                              X                Y                                                                                        =                            ⁢                                                V                  12                                                  V                  14                                                                                                        =                            ⁢                                                Ga                  ×                  Gp                  ×                                      (                                                                  S                        ×                        I                        ⁢                                                                                                  ⁢                        1                                            +                      B                                        )                                                                    Gn                  ×                  I                  ⁢                                                                          ⁢                  2                                                                                                        =                            ⁢                                                S                  ×                                                            Ga                      ×                      Gp                                        Gn                                    ×                  n                                +                                                      Ga                    ×                    Gp                    ×                    B                                                        Gn                    ×                    I                    ⁢                                                                                  ⁢                    2                                                                                                          (        H        )            
It can be seen from Eq. (H) that the emission current I2 is left in the second term of the right side and thus the emission noise cannot be removed.
In the example of FIG. 15, the removal and re-addition of the brightness and the division operation are performed by analog circuits. These processing operations may also be performed by digital computations, in which case measurement of the gain for the brightness that is divided and re-added and adjustments of settings can be carried out automatically.
The conventional noise canceling method described so far has the following problems. First, where a division is performed by an analog circuit (herein referred to as analog division), the emission currents I1 and I2 are in a proportional relationship and the value of the coefficient n is not fixed but varies at all times depending on amounts detected or on various kinds of settings. In order to appropriately divide or re-add the brightness B in Eq. (G), it is necessary to control the factor Gn such that n/Gn is kept constant while maintaining constant Ga and Gp or to control Ga×Gp of the first or second term of Eq. (G) according to variation of n/Gn. Therefore, where the noise canceling function is used, a greater amount of control is needed than where the noise canceling function is not used.
Eq. (B) is about the signal applied to the processing section 160 when the noise canceling function is not used and the Eq. (F) is about the signal applied to the processing section 160 when the noise canceling function is used. In comparing these Eqs. (B) and (F), both agree when 1/(Gn×I2)=1. Therefore, it follows that appropriate brightness is not applied unless 1/(Gn×I2) is equal to unity when the noise canceling function is used. The factor Gn is fixed for the circuit gain but the emission current I2 decreases according to decrease (see FIG. 16) of the emission current of the CFEG 111, as a matter of course. Besides, the value of the emission current I2 will vary when various settings are modified. Therefore, appropriate brightness cannot be removed or re-added unless the factor Gn is controlled appropriately. Brightness which is removed or re-added can be made to assume an appropriate value in the same way by adjusting other gain. In any case, an adjustment is needed whenever some setting or other is modified. In addition to difficulties with adjustments, other problems occur. That is, the value of contrast and the value of brightness are affected depending on whether the noise canceling function is used or not, unless 1/(Gn×I2) is equal to 1. This will increase the number of adjustments. If 1/(Gn×I2) is much greater than 1, the S/N of the image signal will deteriorate. Conversely, if 1/(Gn×I2) is much smaller than 1, the ability to remove the emission noise will be impaired.
Where a division is performed by making use of a digital computation, problems take place similarly to the case of analog division, but the gain can be automatically adjusted prior to final outputting by performing internal computations. However, neither the gain nor the brightness can be varied between the beginning and the end of scanning because an image is observed. Automated adjustments can be performed only in restricted applications. If manual adjustments are made, it can be said that there are no great differences with analog division. In the case of digital division, a division is performed after an analog to digital conversion and so the range of inputtable voltages is narrow. Hence, the digital division provides less versatility than the analog division.
Generally, any brightness adjustments are not made in response to emission current decreases unless the emission current decreases below a preset lower limit, whether the division technique is analog type or digital type. Accordingly, as shown in FIG. 16, where a lower limit giving Gn×I2<1 is set, a division operation consisting of dividing by a value greater than 1, a division operation consisting of dividing by 1, and a division operation consisting of dividing a value less than 1 are performed in succession with the lapse of time from the instant when memory flushing is triggered.